How to resolve the algorithm Van Eck sequence step by step in the ALGOL 68 programming language

Published on 12 May 2024 09:40 PM
#Algol 68

How to resolve the algorithm Van Eck sequence step by step in the ALGOL 68 programming language

Table of Contents

Problem Statement

The sequence is generated by following this pseudo-code:

Using A: Using B: Using C: Using B: Using C: (zero last occurred two steps back - before the one) Using B: Using C: (two last occurred two steps back - before the zero) Using C: (two last occurred one step back) Using C: (one last appeared six steps back) ...

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Van Eck sequence step by step in the ALGOL 68 programming language

Source code in the algol programming language

BEGIN # find elements of the Van Eck Sequence - first term is 0, following  #
      # terms are 0 if the previous was the first appearance of the element #
      #        or how far back in the sequence the last element appeared    #
    # returns the first n elements of the Van Eck sequence                  #
    OP VANECK = ( INT n )[]INT:
       BEGIN
            [ 1 : IF n < 0 THEN 0 ELSE n FI ]INT result; FOR i TO n DO result[ i ] := 0 OD;
            [ 0 : UPB result ]INT pos;  FOR i FROM 0 TO n DO pos[ i ] := 0 OD;
            FOR i FROM 2 TO n DO
                INT j    = i - 1;
                INT prev = result[ j ];
                IF pos[ prev ] /= 0 THEN
                    # not a new element                                     #
                    result[ i ] := j - pos[ prev ]
                FI;
                pos[ prev ] := j
            OD;
            result
       END # VANECK # ;
    # construct the first 1000 terms of the sequence                        #
    []INT seq = VANECK 1000;
    # show the first and last 10 elements                                   #
    FOR i TO 10 DO print( ( " ", whole( seq[ i ], 0 ) ) ) OD;
    print( ( newline ) );
    FOR i FROM UPB seq - 9 TO UPB seq DO print( ( " ", whole( seq[ i ], 0 ) ) ) OD;
    print( ( newline ) )
END

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